FIG. 5 is a perspective view showing a laser diode disclosed in Mitsubishi Denki Giho Vol. 62, No. 11 (1988), pp. 28-31 as an example of a prior art III-V compound semiconductor device having a p-n junction. In FIG. 5, reference numeral 1 designates an n type GaAs substrate. An n type AlGaAs first cladding layer 2 is disposed on the substrate 1. A p type AlGaAs active layer 3 is disposed between the n type AlGaAs first cladding layer 2 and a p type AlGaAs second cladding layer 4 which have lower refractive indices than that of the active layer. The p type AlGaAs second cladding layer 4 has a stripe-shaped ridge structure. A p type GaAs buffer layer 5 and an n type GaAs current blocking layer 6 are disposed on opposite sides of the ridge structure. A p type GaAs contact layer 7 is disposed on the ridge structure and the n type GaAs current blocking layer 6. A stripe-shaped positive electrode 8 is disposed on the p type GaAs contact layer. The conductivity type of the GaAs buffer layer 5 gradually changes from n to p upwardly so that the regrowth surface of the p type AlGaAs second cladding layer 4 does not contact with the n type GaAs current blocking layer, whereby the current blocking characteristic is improved. A negative electrode 9 is disposed on the rear surface of the n type GaAs substrate 1. Selenium (Se) and zinc (Zn) are used as n type and p type dopants, respectively.
A description is given of the operation.
When plus and minus voltages are applied to the positive and negative electrodes 8 and 9, respectively, to bias the p-n junction of the p type AlGaAs active layer 3 and the n type AlGaAs first cladding layer 2 in a forward direction, high concentrations of electrons and holes are injected into the p type AlGaAs active layer 3 from the cladding layers 2 and 4. The injected carriers are confined by a barrier of a heterojunction between the n type AlGaAs first cladding layer 2 and the p type AlGaAs active layer 3 and recombine with a high efficiency in the active layer 3, generating laser light. A greater part of the laser light generated by the recombination of carriers is confined in the active layer 3 due to the difference in refractive indices between the active layer 3 and the cladding layers 2 and 4. At this time, the light emitting region is limited to the center of the active layer 3 due to the current concentrating effect of the current blocking layer 6.
FIG. 6 includes SIMS (Secondary Ion Mass Spectroscope) profiles of selenium (Se) and zinc (Zn) concentrations in the vicinity of the active layer 3. FIG. 7 includes profiles of n type and p type carrier concentrations. The laser diode shown in FIG. 5 is fabricated using an epitaxial growth method and when the AlGaAs first cladding layer 2 is grown, Se is added to a concentration of 10.sup.17 cm.sup.-3 or more as an n type dopant. During the epitaxial growth, if attention is given to the crystal growth considering As, Se atoms fill vacancies V.sub.As, which are originally to be filled with As atoms, and act as donors as represented by the following reaction formula (1). ##STR1##
On the other hand, when the n type AlGaAs first cladding layer 2 and the p type AlGaAs active layer 3 are grown, Zn is added to a concentration of 10.sup.17 cm.sup.-3 or more as a p type dopant. At this time, if attention is given to a crystal growth concerning Ga, Zn atoms fill vacancies V.sub.Ga, which are originally to be filled with Ga atoms, and act as a acceptors as represented by the following reaction formula (2). ##STR2##
As described above, each of Se and Zn is added to a concentration as high as or higher than 10.sup.17 cm.sup.-3 to produce the p-n junction, so that a mutual diffusion occurs at the p-n interface and the profile of the carrier concentration at the p-n interface is not sharp as shown in FIG. 7. In FIGS. 6 and 7, a.u. means an arbitrary unit.
In the above-described laser diode, the sharpness and controllability of the doping profile are poor, which reduces initial performance and reliability of a completed device.
In order to realize a sharp carrier concentration profile at the p-n interface, following methods have been proposed. In Japanese Published Patent Application No. 60-167417, the conductivity type of a semiconductor layer is controlled not by doping impurities but by changing the composition ratio of group III compound semiconductor to the group V compound semiconductor. In Japanese Published Patent Applications Nos. 2-203520 and 3-4517, a vapor phase growth is carried out using trimethylgallium as a dopant gas including a group III element and arsine as a dopant gas including a group V element. During the growth, the ratio of the concentrations of these gases is controlled to incorporate carbon atoms with Ga atoms, Whereby the carbon atoms function as a p type dopant. In Japanese Published Patent Application No. 63-143810, a vapor phase growth is carried out using trimethylgallium as a dopant gas including a group III element and arsine as a dopant gas including a group V element. During the growth, a p type region is formed by incorporating carbon atoms with Ga atoms and an n type region having a desired carrier concentration is formed with As and Se, resulting in a p-n junction.
In the above-described conventional methods, however, it is necessary to change the growth conditions significantly to control the conductivity type of the semiconductor layer, such as a change in the ratio of concentrations of the dopant gases, so that the crystal growth is not favorably carried out at the p-n interface. In addition, when the growth conditions change, vacancies may remain on the growth surface of the substrate without being filled with the carbon atoms. Such imperfect crystal growth and vacancies adversely affect the characteristics of the device, resulting in a poor initial-performance and a poor reliability.